Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, dimensional stability, etc. The PASs are commonly used in a wide variety of technical fields such as electrical equipments, electronic equipments, automotive equipments and packaging materials because they can be molded or formed into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing processes such as extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process in which a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone. As the sulfur source, is generally used an alkali metal sulfide, an alkali metal hydrosulfide or a mixture thereof. When the alkali metal hydrosulfide is used as the sulfur source, the alkali metal hydrosulfide is used in combination with an alkali metal hydroxide.
When at least one sulfur source selected from the group consisting of alkali metal sulfides and alkali metal hydrosulfides is subjected to a polymerization reaction with a dihalo-aromatic compound, a great amount of an alkali metal salt such as NaCl is secondarily produced by a desalting polycondensation reaction. The alkali metal salt remaining in the resulting PAS adversely affects the electrical properties of the PAS because the salt is an electrolyte. When the PAS containing the remaining alkali metal salt is applied to a sealing compound or covering material for electronic parts, the alkali metal salt corrodes electrodes and wirings of the electronic parts, causes disconnection or makes a leakage current great.
The problems caused by the alkali metal salt remaining in the PAS have come to be almost overcome at present. A technique for foaming a high-molecular weight granular PAS in a polymerization step is established as disclosed in, for example, Japanese Patent Publication No. 63-33775 (Patent Literature 1). The granular PAS is easy to be sifted by a screen, and impurities such as alkali metal salts and oligomers can be easily removed from the granular PAS by washing. With respect to a purification method of a PAS collected after polymerization also, is adopted an effective washing method by combining, for example, water washing, washing with an organic solvent, acid washing, etc. In fact, the PAS sufficiently washed is purified to the extent that ash derived from the remaining alkali metal salt such as NaCl is substantially not obtained even when it is burnt. As described above, according to the present state of the art, a PAS substantially containing no remaining alkali metal salt can be obtained.
However, it is difficult even by the present state of the art to reduce the content of halogen atom(s) bonded mainly to terminal(s) of a polymer chain of a PAS. When a sulfur source and a dihalo-aromatic compound are subjected to a polymerization reaction, a PAS with halogen atom(s) bound to one terminal or both terminals thereof is formed. This bonded halogen atom cannot be removed by washing. The sulfur source and the dihalo-aromatic compound are reacted in equimolar amounts to form a polymer. In order to stably perform the polymerization reaction, however, it is desirable to adjust a charged molar ration of the dihalo-aromatic compound to the sulfur source in such a manner that the molar ration of the dihalo-aromatic compound to the sulfur source becomes somewhat excessive. On the other hand, if the molar ration of the dihalo-aromatic compound to the sulfur source becomes too excessive, a PAS with halogen atoms bonded to both terminals of its polymer chain is liable to be formed.
The requirement of halogen-free regulation on the whole polymer product from environmental groups is heightened in addition to the requirement of halogen content reduction in PASs by electronic part makers from the viewpoint of the performance of electronic parts, and PAS makers bear the responsibility of satisfying these requirements. In order to reduce the halogen content in a PAS, it is essential to reduce the content of the bonded halogen atoms in addition to the reduction of the remaining alkali metal salt.
As a method for reducing the content of the bonded halogen atoms in the PAS, Japanese Patent Application Laid-Open No. 62-106929 (Patent Literature 2) proposes a method of subjecting poly(phenylene sulfide) (PPS) and a mercapto group-containing compound or a salt thereof to a heat treatment in a solvent capable of dissolving the poly(phenylene sulfide). The method disclosed in Patent Literature 2 includes a step of collecting, washing and drying the PPS after sodium sulfide and p-dichlorobenzene are polymerized in N-methyl-2-pyrrolidone. Patent Literature 2 shows an experimental example that the PPS dried, the mercapto group-containing compound or the salt thereof and the solvent were charged into an autoclave to react them under heating. Patent Literature 2 describes that the contents were poured into water after completion of the reaction under heating, washing with hot water and filtration were conducted repeatedly, washing with methanol was then conducted, and vacuum drying was conducted to isolate a polymer.
According to the method described in Patent Literature 2, PPS whose content of a bonded chlorine has been greatly reduced can be obtained. However, the method of Patent Literature 2 is not efficient because the PPS must be caused to further react with the particular reaction reagent at a high temperature over a long period of time in the solvent after the PPS is collected from the polymerization reaction system and purified after polymerization, and a purification step including filtering, washing and drying is required after the reaction. In short, the method described in Patent Literature 2 is complicated in operation and moreover also low in energy efficiency.
Japanese Patent Application Laid-Open No. 5-163349 (Patent Literature 3) proposes a process for producing a PAS having a low bonded chlorine content by subjecting a cyclic arylene sulfide oligomer to ring-opening polymerization under heating in the presence of a ring-opening polymerization catalyst. However, the process disclosed in Patent Literature 3 requires to extract a 7- to 15-mer cyclic phenylene sulfide oligomer from a PPS formed after a polymerization reaction of sodium sulfide with p-dichlorobenzene in an organic amide solvent by Soxhlet extraction using methylene chloride as a solvent. Therefore, the process of Patent Literature 3 is not a process suitable for production of the PAS on an industrial scale.
Citation List
Patent Literature
    Patent Literature 1: Japanese Patent Publication No. 63-33775 (corresponding to U.S. Pat. No. 4,645,826)    Patent Literature 2: Japanese Patent Application Laid-Open No. 62-106929 (corresponding to U.S. Pat. No. 4,820,801)    Patent Literature 3: Japanese Patent Application Laid-Open No. 5-163349 (corresponding to U.S. Pat. No. 5,384,391)